Oil shear brake and clutch units have been developed to eliminate the problems associated with the dry friction type of units. Properly designed oil shear clutch or brake drives offer the advantage of little or no wear of the friction plates in the disk stacks and no fading. These oil shear units thus provide a more precise operation of the machine tool and dramatically increase the machine tool's up-time. The oil film between the adjacent friction plates carries the heat generated by the starting and stopping of the machine tool away from the friction plate stacks. This removal of heat offers the advantage that there is now no practical limit in the disengage/engage rate or in the speed of the input device.
Oil shear clutch units are utilized to intermittently transfer rotational power from a continuously rotating input shaft to an output shaft. The output shaft is connected to the input of a machine tool. The clutch unit is normally operating in a disengaged condition. The input shaft is rotating with respect to the output shaft and there is no power being transmitted through the clutch unit. When a control system gives a command to operate the machine tool, the clutch unit is engaged to lock the input shaft to the output shaft and transmit power through the clutch unit.
Typical clutch units can be engaged electrically, pneumatically or hydraulically. The; choice of an electric clutch versus a pneumatic clutch versus a hydraulic clutch is sometimes determined by the availability of electrical, pneumatic or hydraulic power and sometimes the design choice for the brake unit, is dictated by the application or machine tool to which it is being mated. When the driving torques or power being transferred through the clutch unit increase, electrical operation of the clutch is no longer a viable option. This is due to the clamping loads required between the friction plates and the required electrical components needed to generate these loads. Thus, higher power clutch units are typically pneumatically or hydraulically actuated.
When considering the choice between pneumatic and hydraulic operation of the clutch, the choice can be dictated by the availability of a source of compressed air or a source of pressurized hydraulic oil. When considering compressed air as the actuating medium, the lower the pressure of the available compressed air, the larger the area for the piston which generates the required load. Thus, unless a high pressured air source is readily available, the choice for the design of the higher powered clutch unit will be hydraulic actuation.
When considering hydraulic actuation, the source of the pressurized hydraulic fluid can be external to the clutch unit or the clutch unit can incorporate an oil pump which supplies the necessary pressurized hydraulic fluid. For oil shear clutch units, the integration of the oil pump into the clutch unit allows for the sharing of an oil sump because the oil shear clutch units typically include an oil sump for lubricating bearings, friction plates and other moving components.
One consideration when developing clutch units with integrated pressurized hydraulic fluid supplies is the direction of rotation of the clutch unit. Typically oil pumps are unidirectional and thus consideration must be given to the direction of rotation. Preferably, a clutch unit should be designed to operate in both a clockwise direction and a counterclockwise direction with minimal changes to the clutch unit in order to properly function in either direction.
Thus, the continued development of clutch units have been the development of hydraulic fluid management systems which allow the operation of the clutch unit in both rotational directions without having to manually adapt the clutch unit.